I have a Canon T6i and I am looking to buy a new lens, I am mostly interested in product photography, sharpness and image quality is super important to me. I am currently using a kit lens which is Canon EFS 18-135mm and I am thinking about buying Sigma 24-105mm f/4 DG OS HSM Art and it's very expensive so I need to get some advice from an expert before buying it. Is it worth it or not? Am I ganna see a big difference in the quality? What is your advice to me?

For product photography do you have a specific focal length you like, say 50mm on you're cropped sensor? I ask because it might be better to look at a fixed focal length lens that also allowed macro focusing.

You really need to decide as to what aspect of quality the EF-S 18-135 is not providing before you can decide which lenses might be better. If you are think sharpness, there is more to sharpness than the lens.